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Day 26 - Static
Fire Tests

This photo from last week is what prompted
us to carry out these tests

Setting up cameras for the experiment.

... and a couple more.

It is a good idea to wear ear protection, as
it can be pretty loud.

Firing the rocket by hand.

Following are frames from
the slow motion video.

Just prior to launching test #1.

Initial bubble blow-through, extends all the
way to the nozzle.

Jet of water from upper bottle creates a lot
of bubbles in the lower bottle.

Lots of bubbles causes the spray to widen.

When bubbles reduce then flow from the
nozzle improves.

During the air pulse, some remaining water
is forced up the sides of the bottle.

The cool fog inside the rocket after a test
firing.

Measuring the inside and outside
temperatures of the rocket. The thin wire is the
thermocouple.

A newer rocket rebuilt from the
last launch attempt that failed on
the launch pad. Fixed to launcher.

Team Members at
Launch Event:
PK, GK, AK, John K and Paul K.Number
of firings: 4

Today was a slightly different launch
day. After reviewing last weeks photos and
videos we noticed that both J4 II and Polaron
III exhibited unusual nozzle spray behaviour
and in one photo the water in Polaron III
appeared cloudy just after takeoff. So we
decided to perform some static fire tests to
see what was going on.

Flight Day Events

During the week I managed to figure out
(well actually read the manual) how to use our
digital still camera to record video at 60
frames per second. Because of the faster
frame rate it only records at a resolution
of 320 x 240 but that was enough for our needs.

We set up Polaron III on the new launcher
and tied it down so it wouldn't fly away. We
knew that we didn't need to weigh down the
launcher, because the rocket's thrust would
be pushing against it and therefore
providing an equal but opposite reaction
force. But we did anyway, just in case
Newton was asleep at the wheel on the day.

We set up a few cameras around and did
the following experiments:

Static Fire Test #1

We filled the rocket with 2.5
L of water. There was about 600ml was in the top
bottle, and about 1.9L in the bottom bottle.
With an air gap in the top of the lower
bottle.
This water configuration is quite easy to
achieve, just pour the water in and let it
partially fill the next bottle. As the top
bottle is closed off the small
Robinson coupling hole mostly keeps the
water coming through to the bottom bottle
and hence leaving an air gap. The nozzle was
a 9mm Gardena nozzle.

(If the video does not play, try
the latest
Flash player from Macromedia)

We pulled the hose off the rocket and
watched what happens to the water inside the
bottle and the corresponding spray from the
nozzle. The video clearly shows the jet of
water from the top bottle dragging air with
it penetrating all the way through the water
in the lower bottle. This starts a process of
generating a great many bubbles in the water
against the nozzle and progresses up
the bottle until the entire bottle is filled
with bubbles so that it is milky in
appearance. The nozzle spray also correspondingly
widens the same way as we saw on the last
launch day. The air bubbles in the water
must be expanding as they exit the nozzle,
and hence the wide spray.

After a while when the water runs out in
the top bottle and water is only being
pushed out of the bottom bottle, the nozzle spray
narrows again and the water becomes a little
more clear, although the surface is very
rough and there is a substantial number of
bubbles in it.

As the water is about to run out, the and
the air
pulse happens, the airspeed inside the
rocket increases significantly. The
air flow is such that the remaining water actually gets pushed up
the sides of the bottle and is kept there
until the air runs out and the water then
just drains out of the nozzle. It wasn't a lot
of water but it was an unexpected result.

Static Fire Test #2

The second test involved only filling the
lower bottle with 1.9L of water so that
there was no water in the upper bottle. We
used exactly the same pressure as the
previous test.

Results

On launch the blow-through also happened,
but the blow-through bubble quickly
retracted again to the surface and the water
stayed relatively bubble free for the
duration of the burn. The nozzle spray was also relatively
clean.

(If the video does not play, try
the latest
Flash player from Macromedia)

The next two tests were used to measure
the temperature of the bottles inside and
outside as they were
filling and just after they emptied. No
water was used in these experiments. The
ambient temperature was 22 degrees C. All
temperatures were measured with a tiny thermocouple connected to multimeter that
can measure temperature. The thermocouple
can measure rapid changes in temperature.

When measuring the skin temperature we
just placed the sensor onto the bottle, but
probably should have been shielded from the
environment.

Static Fire Test #3

The rocket was filled to 50 psi and the
outside skin temperature was measured during
filling to be 25 degrees C. +3 above
ambient.

When the air was let out, the
thermocouple was placed inside the rocket and the air
temperature was measured at 16 degrees C. -6
below ambient.

(If the video does not play, try
the latest
Flash player from Macromedia)

Static Fire Test #4

The rocket was filled to 100 psi and the
outside skin temperature was measured during
filling to be 27 degrees C. +5 above
ambient.

When the air was let out, the
thermocouple was placed inside and the air
temperature was measured at 8 degrees C. -14
below ambient.

Lessons learned

Our experiment confirmed that the
wide water spray observed last week was
due to expanding bubbles rather than a
faulty nozzle/seal as previously believed.

Designing a better coupling might mean
that the rocket does not produce this "blow
through" and allow the water to come out
more evenly and hence more efficiently.

Water flows up the sides of the lowest
bottle during the air
pulse. Minimising this should also allow
better flight efficiency.

To get a more efficient burn there
should not be any water in the upper
bottle above the coupling. Water often
ends up there if you fill the rocket too
quickly with air.

The temperature measurements indicated
that there is unlikely to be much effect
on the PET bottle material with the
pressures we regularly use.

Things to consider

If the bubbles are produced in an
efficient manner it may be possible to use
that to an advantage with an expanding
nozzle.
Antigravity
Research used detergent
to create foam inside the rocket with an
expanding nozzle to make the rocket more
efficient. When perfected a technique based
on the static fire test #1 may be used to
generate the same effect without the
viscosity penalty of using detergent.

If the temperature of the air inside the
rocket drops below freezing at higher launch
pressures, it may be likely that the during
the air pulse tiny water droplets exiting
could be freezing and exiting as ice or
snow?

Notes

These were static tests that do not take
into account the acceleration of the rocket,
so the water may behave differently, but
from last weeks tests we see that even
accelerated rockets behaved in a very
similar manner.

We purposefully recorded the rocket
video sideways so that we could see more
of the action.

What's Next?

We will try to improve
the coupling to prevent this "blow-through".
We already have a simple idea that may fix
this. More on that next time.

We will need to do further tests
with a different sized nozzles, and bottle
capacities and shapes.

We may try to get a consistent bubble
forming scheme and use an expanding nozzle
to see if it makes a measurable
difference.

Flight Record

Test

Rocket

Pressure (PSI)

Notes

1

Polaron III

110

With some water in the upper bottle,
water in lowest bottle exhibited a
significant amount of bubbles which
caused the nozzle spray to widen.

2

Polaron III

110

With no water in the top bottle the
blow-trough also happened but
settled quickly, for an efficient
burn.

3

Polaron III

50

Temperatures measured inside and
out.

4

Polaron III

100

Temperatures measured inside and
out. Higher pressure made a
significant difference on the
temperature difference.